When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus.” For example, if a swine origin influenza A H3N2 virus is detected in a person, that virus will be called an “H3N2 variant” virus or “H3N2v” virus.

Swine are highly susceptible to a variety of flu viruses (human, swine, avian) - and are viewed as excellent `mixing vessels’, allowing viruses to reassort into new hybrid strains.

While most reassortant viruses are evolutionary failures, the concern is that over time another swine variant virus might emerge – as did the 2009 H1N1 virus – and start another human epidemic or pandemic.

One strategy to try to prevent that from happening is limiting the number of humans that are exposed to these variant viruses, depriving the virus of opportunities to adapt to human physiology. To that end, the CDC has issued the following advice for those planning to visit the fair:

For the past two years the CDC has also provided extended advice to exhibitors of swine, and those in charge of the venues, on their Guidance Associated with Fairs website.

It is estimated that 150 million people visit agricultural fairs each year in North America. Agricultural exhibitions provide valuable educational venues for the public. Equally important, livestock shows are an important learning opportunity for thousands of 4-H and FFA youth across the United States. For these youth, exhibiting at their county or state fair is the culmination of many months of work dedicated to the care and training of their animal.

Pigs can be infected with human, swine and avian origin influenza A viruses. While rare, influenza A viruses can spread from pigs to people and from people to pigs. Transmission usually requires close contact between pigs and people. This has happened in different settings, including livestock shows. When people are infected with swine origin influenza A viruses, it is termed as a variant virus infection and denoted with a “v” after the subtype (e.g. H3N2v).

In the past 5 years, cases of influenza A H1N1v, H1N2v and H3N2v have been associated with swine exhibitions. Between 2011 and 2013, 340 human cases infected with H3N2v were reported from 13 states. The largest outbreak occurred in 2012 when a total of 309 human cases of H3N2v flu were identified.

It's possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur. While there is no evidence at this time that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it's possible that this virus may become widespread. Illness associated with H3N2v infection so far has been mostly mild with symptoms similar to those of seasonal flu. Like seasonal flu, however, serious illness, resulting in hospitalization and death is possible. In 2012, for example, 309 infections with H3N2v were detected. Of these cases, 16 people were hospitalized and one of these people died. Most of the people who were hospitalized and the person who died had one or more health or age factor that put them at high risk of serious flu-related complications.

People at high risk of serious complications from seasonal influenza and H3N2v include children younger than 5, people with certain chronic conditions like asthma, diabetes, heart disease, weakened immune systems, pregnant women and people 65 years and older. CDC has issuedguidance for people attending fairs where swine might be present this fair season, including additional precautions for people who are at high risk of serious flu complications. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not. Most cases of H3N2v infection have occurred in children who have little immunity against this virus.